Cancer pain is not necessarily correlated with spinal overexpression of reactive glia markers

Bone cancer pain is a common and disruptive symptom in cancer patients. In cancer pain animal models, massive reactive astrogliosis in the dorsal horn of the spinal cord has been reported. Because astrocytes may behave as driving partners for pathological pain, we investigated the temporal development of pain behavior and reactive astrogliosis in a rat bone cancer pain model induced by injecting MRMT-1 rat mammary gland carcinoma cells into the tibia. Along with the development of bone lesions, a gradual mechanical and thermal allodynia and hyperalgesia as well as a reduced use of the affected limb developed in bone cancer-bearing animals, but not in sham-treated animals. Dorsal horn Fos expression after nonpainful palpation of the injected limb was also increased in bone cancer-bearing animals. However, at any time during the evolution of tumor, there was no increase in glial fibrillary acidic protein (GFAP) immunoreactivity in the dorsal horn. Further analysis at 21 days after injection of the tumor showed no increase in GFAP and interleukin (IL) 1β transcripts, number of superficial dorsal horn S100β protein immunoreactive astrocytes, or immunoreactivity for microglial markers (OX-42 and Iba-1). In contrast, all these parameters were increased in the dorsal horn of rats 2 weeks after sciatic nerve ligation. This suggests that in some cases, bone cancer pain may not be correlated with spinal overexpression of reactive glia markers, whereas neuropathic pain is. Glia may thus play different roles in the development and maintenance of chronic pain in these 2 situations.